Comedy in a scene should arise out of character and situation organically, not be forced in from some arbitrary bag of tricks or memory of punchlines. All of the techniques listed below are ways of forcing a laugh -- often through inserting a cheap joke -- and may actually prevent you from creating a genuinely funny scene by constantly denying the reality of the situation. Sure, at some time or another, everybody resorts to them in a pinch, and there are even games designed specifically to support one or the other crutch. But know what they are and only use them with caution. A cheap laugh may feel good while you're getting it, but you'll feel all dirty and hate yourself in the morning.
Gags are funny bits that don't really fit but you shoehorn them in, anyway. Puns. Old joke lines ("and you know how painful that can be...," "If you hum a few bars I can fake it," etc.). Physical bits that don't really belong there but you thought a fall would get a laugh. It's anything that removes you from the scene so the audience is laughing at/with you the actor, not you the character. Stay in character.
A callback is a reference to something that happened earlier in the show, usually in another scene, that you work into this scene so you can try and use the memory of the earlier bit to get a laugh here. Do it more than once and it becomes a running gag. Do it one too many times or too clumsily and it gets a groan instead of a laugh and brings whatever you tried to force it into to a dead stop. Use callbacks carefully.
Torturing the actor/pimping/denial.
Sure, some games (translation games, What Are You Doing, That'll Be Charley Now, etc.) are built on pimping, and those are times when it's appropriate. But pure pimping (which means trying to get a fellow actor to do something they are not good at or willing to do in order to get a cheap laugh through their ineptitude or embarassment rather than the scene at hand) for the hell of it is just going to break the reality of a scene and make both of you look bad. Some actors think that the general rule against denial in a scene means they get carte blanche to bully their castmates into being their puppet ("Hey, sing me that song you used to sing!" "How did that dance go again?" "Daddy, carry me piggyback!"). Sorry, but you can accept the reality of a scene without carrying out every malicious request made by someone who just wants to screw with you. Demanding that another actor do what you say in the name of not denying doesn't make you a stickler for quality improv, it just makes you an asshole.
The "Funny" suggestion.
Beware of seeking strange or funny sounding suggestions for characters, scene locations, etc. from the audience. Sometimes if you pick something too weird to start with you're stuck in a one joke premise and you've got nowhere to go from there. On the other hand, if you start out with a boring sounding suggestion you've got room to build, to filter it through your own comedic sensibility and impress the audience when you make it into something funny. Creating the humor is your job, not theirs.
Current events, local names, and pop culture personalities.
I'm not saying don't be current, I'm just saying have something to actually say about events instead of just throwing out funny names and tired memes (see Shatner below).
Don't play types. Play people.
Learn to do improv without profanity and innuendo and you'll be able to handle it when appropriate. If you think you have to use it to be funny, you don't really know how to be funny. You just know how to get nervous laughter through shock value -- and when the shock wears off, you won't be funny anymore.
We've all seen (and many have done) a scene based on Star Trek that devolves into a really... really... tortured... dramatic pause-laden then sped up and stacatto Bill! Shatner! As! Kirk! (Wait for it...) Impression! Maybe it even got a laugh. So, what's wrong with that?
What's wrong with it is that it has been done. To death. By everyone. They've even done it on Family Guy, which is a sure sign that the bit exhibits creative bankruptcy. Not only is it old and stale but, worse, it isn't your bit. In improv comedy we are uniquely situated to create something new and startling in the moment, exercising our own creativity and making our own statement, and here you want to throw that chance in the dumpster for a cheap, recycled laugh. You're better than that. Don't steal somebody else's material, and that includes impressions. Show us something new about Shatner (or do somebody else from the Trek universe for god's sake -- amaze us with your command of minor characters and roll out that killer Lt. Leslie). If it's a bit you saw on Family Guy, or Saturday Night Live, or Whose Line Is It, Anyway?, or done by some other guy in your improv group last week -- if it isn't your bit -- don't do it. Come up with your own, fresh take. No Shatner.